1. Field of the Invention
The present invention relates to a method for producing a welded resin material by joining a resin member having transmissibility to laser light and a resin member having absorptivity to laser light, and to a welded resin material.
2. Description of the Related Art
Such a method has been well known in the art that a resin member having transmissibility to laser light having a specific wavelength and a resin part having absorptivity to laser light having the same wavelength are superimposed and irradiated with laser light from the side of the transmissive resin member to weld them. The principle of the method will be described briefly. Laser light passes through the transmissive resin with substantially no absorption and is absorbed in the vicinity of the surface of the absorptive resin member. The energy of the laser light thus absorbed is converted to heat, which heats the surface of the absorptive resin member. The vicinity of the surface of the transmissive resin member, which is in contact with the surface of the absorptive resin member, is also heated through heat transfer. As a result, a molten layer is formed at a part where the transmissive resin member and the absorptive resin member are in contact with each other, and the molten layer is solidified to weld the resin members.
As apparent from the principle, the contact property between the transmissive resin member and the absorptive resin member is important upon welding. In the case where the contact is insufficient, heat transfer from the absorptive resin member to the transmissive resin member become insufficient to cause failure in joining. In order to ensure the surface contact property between the transmissive resin member and the absorptive resin member, in general, the resin members are in contact with each other under pressure at the contact part and irradiated with laser beam.
The molten layer formed at the contact part is applied with the contact pressure upon welding as described above, and therefore, a part of the molten resin is excluded outside the contact part. The molten excluded resin grows in a strip or brush form associated with progress of melting if no particular measure is instituted, and is liable to be solidified in an incisive shape after melting.
The excluded resin solidified in the incisive shape is liable to be dropped off upon application of vibration or impact, and causes a problem in contamination of environments, in which the product having the molten part is used or transported. In the case of a product having an internal space having a precision component, such as an electronic part, housed therein, there is a problem of damaging the precision part due to impact of the excluded resin dropped in the internal space. Furthermore, even though the excluded resin is not dropped off, the incisive part exposed externally disfigures the product.
Under the circumstances, the method disclosed in JP-A-2004-358697 as the first method for producing a welded resin material includes the steps of superimposing a resin member having transmissibility to laser light and a resin member having absorptivity to laser light to form a closed space at the contact interface between the resin members, and radiating laser light through the closed space from the resin member having transmissibility to laser light to fill the closed space with a molten material obtained by melting the surrounding resin, whereby the resin members are welded. In order to fill the closed space with the molten material for welding, however, welding and sealing cannot be attained unless the closed space is filled with the molten material. Accordingly, a large quantity of energy of laser light and a prolonged welding time are required, and there is a possibility of destabilizing the welding property. Furthermore, the density of the surrounding resin is decreased to deteriorate the durability when the surrounding resin of the closed space is melted excessively.
In the method as the second method for producing a welded resin material, there is an attempt to reduce the amount of the excluded resin formed. That is, the energy of laser light radiated onto the contact part is restricted to melt only the surface layers of the transmissive resin members and the absorptive resin members for welding. In this method, the amount of the formed excluded resin can be reduced, but there is a risk of failure on joining when only a slight gap is formed at the contact part. Accordingly, the contact surfaces of the resin members are necessary to have high planarity before welding, which often exceeds beyond the planarity that can be obtained by the ordinary resin molding techniques. Thus, there is a problem of deterioration in productivity when the excluded resin is suppressed from being formed.